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Date: Mon, 11 Feb 2013 10:59:09 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, LKML <linux-kernel@vger.kernel.org>,
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        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [ANNOUNCE] 3.8-rc6-nohz4
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* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> > I'm worried about the proliferation of not easily separable 
> > config options. We already have way too many timer and 
> > scheduler options to begin with.
> 
> Like Steve said, this is for overhead reasons. The syscall 
> uses the slow path so that's ok. But we add a callback to 
> every exception, irq entry/exit, scheduler sched switch, 
> signal handling, user and kernel preemption point. This all 
> could be lowered using static keys but even that doesn't make 
> me feel comfortable with this idea.
> 
> Moreover, for now this is going to be used only on extreme 
> usecases such as real time and HPC. If we really have to merge 
> this into an all-in-one nohz kconfig, I suggest we wait for 
> the feature to mature a bit and prove that it can be useful 
> further those specialized workloads, and also that we can 
> ensure it's off-case overhead is not significant.

I have no problems with making it an option initially - as long 
as the options are logically named and interconnected.

In terms of overhead, a big plus is the reduction in user-space 
execution overhead. At HZ=1000 we easily have 0.5%-1.0% overhead 
currently. That is a *lot* of overhead if the box does mostly 
user-space execution - which most boxes do, both servers and 
desktops - not HPC systems.

Thanks,

	Ingo